1. Field of the Invention
The invention relates to a backlight device, and particularly to a system for driving a plurality of lamps used in backlight modules of liquid crystal displays.
2. Description of Related Art
Discharge Lamps, especially Cold Cathode Fluorescent Lamps (CCFLs), are used as light sources for liquid crystal display (LCD) panels. Typically, the CCFLs are driven by a plurality of inverter circuits. An inverter circuit provides alternating current (AC) signals to the CCFLs.
For large-size LCD panels, two or more CCFLs are typically required to provide sufficient luminance. The inverter circuit employed by the large-size LCD panels accordingly includes a plurality of outputs to respectively provide AC signals to each CCFL. When a short-circuit or break causes a fault on any output of the inverter circuit and a corresponding CCFL fails, the LCD panel loses its luminance symmetry. Due to lack of fault detection capabilities on the inverter outputs, the backlight inverter circuit fails to provide proper protection functions when faults occur.
Additionally, faults or abnormalities may occur in lamps or other elements and affect the entire display system. For example, if a lamp breaks, the resulting open circuit stops current flow therethrough. Impedance in a lamp, however, may also change with its age and affect operation and luminance thereof. That is, based on backlight use, some lamps may age and gradually go unqualified. As more lamps are employed in a display as backlights, impact of lamp failure to the system electric characteristics (e.g. voltage and/or current) become less perceptible, which makes lamp abnormality detection more difficult. Further, voltage and current required to drive a discharge lamp varies with impedance thereof, which changes not only with age of the lamp but also the states of the lamp. For example, voltage required to start up a discharge lamp from its off state is higher than needed to drive the discharge in the on state. Thus, detecting lamp abnormalities by comparing electric characteristic thereof with a static threshold value may be unreasonable. This further complicates fault detection on lamps.